Method and apparatus for synchronization of a receiver to a transmitter

ABSTRACT

In a method for synchronization of a receiver to a transmitter which periodically transmits a sequence which is known in the receiver, a subset of possible synchronization times is determined in a first selection process by repeated correlation of the received signal with the known sequence and comparison of the correlation responses, as calculated for the correlations, with a threshold value. The threshold value is in this case varied adaptively as a function of at least one parameter which is measured during the correlations.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/DE03/02680, which was notpublished in English, which claims the benefit of the priority date ofGerman Patent Application No. DE 102 41 690.7, filed on Sep. 9, 2002,the contents of which both are herein incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The invention relates to a method and an apparatus for synchronizationof a receiver to a transmitter, with the transmitter periodicallytransmitting a sequence which is known in the receiver.

BACKGROUND OF THE INVENTION

In many areas of communications technology and in particular in mobileradio systems, a receiver must be synchronized to a transmitter before aconnection is set up. For this purpose, the transmitter generallyperiodically transmits a specific sequence, which is known in thereceiver and is also referred to as a sync word. This sequence issearched for in the incoming data stream in the receiver. When thesequence is found in the incoming data stream, the soughtsynchronization time is obtained from the timing of the detectedsequence.

The data stream is typically subdivided in the transmitter intoso-called slots (time slots) with a fixed number of bits (which are alsoreferred to as chips when the spreading code method is used). The syncword which must be identified by means of a suitable method in thereceiver is located at the start of a slot such as this. The expressionslot synchronization is used in this case.

FIG. 1 shows a known procedure for determination of the slot start bymeans of a matched filter (search filter). The sampling time period isannotated T, while k denotes the time index (that is to say the discretetime). The sample values of the in-phase and quadrature components ofthe baseband signal x(kT) are supplied as an input signal 1 to anamplitude control means AGC (Automatic Gain Control) 2. The signal 3emitted from the AGC 2 is passed to a low-pass filter TP 4. A signal 6with a real amplitude is produced by magnitude formation (addition ofthe squares of the signal components) in a unit 5. This signal 6 isfiltered by a matched filter 7. The impulse response of this filter 7corresponds to the complex-conjugate sync word reflected in the timedomain. The matched filter 7 produces a result g(kT) for each samplevalue x(kT). The maximum value occurs, in a similar way to that in thecase of a correlator, at the filter output when the slot start has beenfound (minus the latency of the filter 7). The filtered signal g(kT) forall the sampling time kT, for example k=1, . . . , 2560, . . . , 5120,provided that 5120 samples are taken within one slot, is thus referredto as the correlation response 8.

A search for the maximum is carried out in a unit 9 within anobservation interval, which corresponds to the slot duration, in orderto detect correlation peaks in the correlation response 8. The maximumU_(max) and the associated time T_(max) are passed to an evaluation unit10. The evaluation unit 10 compares the detected maximum U_(max) with aspecific, previously defined threshold value. The evaluation result 11is signalled to a decision maker 12. If the threshold value has beenexceeded, the decision maker 12 decides that a sync word has beendetected at the time T_(max).

Since the mobile radio channel is not a static channel, it is notsufficient to carry out the method described above for only a singleslot. A number of slots typically have to be processed in order to makean error-free decision on the slot boundaries. In consequence,intermediate results of the correlation responses 8 must be stored overa number of slot intervals and must be accumulated in the evaluationunit 10 before comparison with the threshold value. The threshold valuelevel determines the yield and confidence of the method. The higher thepreset threshold value in the evaluation unit 10 the fewer sync wordsare detected but, furthermore, the lower is the probability of anincorrect decision based on a sync word being simulated by interference.

The slot length in the UMTS (Universal Mobile Telecommunication System)Standard is 2560 chips (an in-phase and quadrature component in eachcase). An oversampling factor of 2 is normally used. Up to 10240 samplevalues therefore have to be stored, based on a typical resolution of 8bits per slot.

A multistage evaluation method may be used in order to reduce the amountof memory. The principle of multistage evaluation is based on the ideaof a subset of possible times for the slot start in different (mobileradio) cells being determined first of all during a initial selectionprocess, for each slot. Only the times included in this subset areprocessed further and are possible candidates for the slot start of oneor more cells. The slot start of each cell is then determined in asecond selection step, which is more accurate and has a higherresolution than the first selection step.

With regard to the example of the UMTS standard, it is possible, by wayof example, to provide for the 5120 correlation values emitted from thematched filter for each slot (for one signal component with doubleoversampling) to be restricted to a subset of, for example, 1280correlation values. This initial selection process can be carried outwith relatively little effort, but itself reduces the possiblesynchronization times by a factor of 4. The second selection steprequires considerably greater accuracy (since only that time index atwhich the slot start is located must be determined from the 1280 timeindices), but it need be carried out only for the preselected 1280 timeindices k.

One difficulty with a multistage method such as this is that the twoselection steps (initial selection and final selection) must be matchedto one another as accurately as possible. In practice, the maximum sizeof the subset which can be processed in the second selection step islimited by hardware requirements (memory size, clocking etc). If thenumber of possible time indices in the initial selection process isgreater than this subset, it is not possible to take account of all thepreselected time indices in the second selection step. This results inthe loss of information which was obtained in the first selection step.If, on the other hand, the number of time indices calculated in thefirst selection step is considerably less than the size of the subset(for example 1280 correlation values), the capacity of the secondselection step is not utilized. Furthermore, during the initialselection process, there is a risk of this preselection process beingexcessively strict bearing in mind the low accuracy, thus likewisereducing the amount of information available for the second selectionstep. It is therefore important for the initial selection process to becarried out so as to just exhaust the capacity of the subsequent secondselection step.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentone or more concepts of the invention in a simplified form as a preludeto the more detailed description that is presented later.

The invention is directed to a method for synchronizing a receiver to atransmitter, which ensures that a number of preselected possiblesynchronization times obtained in a first selection step is matched to apredetermined capacity of a second selection step. A further aim of theinvention is to provide an apparatus for synchronizing a receiver to atransmitter, which allows capacity matching between a first and a secondselection step.

According to one embodiment of the present invention, a set of possiblesynchronization times for one or more cells is determined in a firstselection step by repeated correlation of the signal received by thereceiver with the known sequence and comparison of the correlationresponses, as calculated for the correlations, with a threshold value.In a second selection step, the actual synchronization time of one cellor the actual synchronization times of two or more cells is or aredetermined from the set of possible synchronization times.

In this exemplary embodiment, the threshold value is adaptively variedin the first selection step as a function of at least one parameterwhich is measured during the correlations. The threshold value is variedin such a way that the set of two or more possible synchronization timesdetermined in the first selection step always comprises virtually thesame number of possible synchronization times, with the number beingpredetermined. This guarantees that the second selection step is alwaysbased on an essentially constant number of possible synchronizationtimes, thus resulting in optimum matching between the first and thesecond selection step.

The first selection step preferably comprises the following: (a1)comparison of the correlation values of a correlation response with thethreshold value; (a2) notation of the times at which the correlationvalues of the correlation response exceed the threshold value; (a3)measurement of the at least one parameter; (a4) setting of the thresholdvalue as a function of the parameter for the next correlation process;(a5) repetition of steps (a1) to (a4) for further correlation responses;and (a6) determination of the set of possible synchronization times asthose times at which the adaptive threshold value has been exceeded apredetermined number of times.

It should be mentioned that the set of possible synchronization times,can be fixed by presetting the number of times the threshold value mustbe exceeded in order to include the appropriate synchronization time inthe set. However, this definition is too coarse on its own (that is tosay without the threshold value adaptation according to the invention).For example, a situation may occur in which a (fixed) threshold value isexceeded m times for a given number of correlations (Ncor) for acomparatively large number of synchronization times (for exampleconsiderably greater than 1280) and where the same threshold value isexceeded m+1 times only for a comparatively small number of possiblesynchronization times (for example considerably less than 1280). Bothcriteria (the threshold value being exceeded m times and the thresholdvalue being exceeded m+1 times) are then unsuitable for production of aset of possible synchronization times whose size is about 1280.Effective capacity matching between the first selection step and thesecond selection step is achieved by the adaptive threshold valueadjustment from one correlation process to the next in accordance withthe present invention.

The at least one measured parameter which is used for adaptiveadjustment of the threshold value is preferably characteristic of thestatistics of the correlation response. In this situation, a firstadvantageous method variant is characterized in that a first parameteris a measure of the standard deviation of the correlation response. Asecond advantageous method variant is characterized in that a secondparameter indicates the number of times at which the correlation valuesof the correlation response exceed the current threshold value in theindividual matched filter runs. Taking account of the statisticalproperties of a correlation response for the adjustment of the thresholdvalue for the evaluation of the next correlation response ensureseffective control of the number of possible synchronization timesselected in the first selection step.

One advantageous procedure for adjusting the threshold value from onecorrelation process to the next is characterized in that the thresholdvalue is increased by a first threshold value offset when the number oftimes at which the correlation values of the correlation response exceedthe threshold value is greater than a first predetermined number.Further, the threshold value is reduced by a second threshold valueoffset when the number of times at which the correlation values of thecorrelation response exceed the threshold value is less than a secondpredetermined number, and in that, for example, the first thresholdvalue offset and/or the second threshold value offset increase as thedistribution width and standard deviation of the correlation values ofthe correlation response rises. This means that the threshold valuecorrection becomes greater as the distribution of the correlationresponse becomes flatter. In contrast, if the distribution width andstandard deviation of the correlation response are small, only minorthreshold value corrections are carried out from one correlation processto the next.

One exemplary embodiment of the invention is characterized in that thecorrelations in the first selection step are evaluated on the basis ofhard-decided (that is to say dual) signal values. This considerablyreduces the computation complexity, although a relatively high degree ofinaccuracy must be expected owing to the minimal resolution of thesignal values (as is desired of the 2-stage selection method usedaccording to the invention).

The second selection step may include any desired further processing ofthe possible synchronization times determined in the first selectionstep. According to one embodiment variant of the invention, the secondsynchronization step likewise includes a correlation step. This meansthat the signal obtained at the possible synchronization timesdetermined in the first synchronization step are once again correlatedwith the known sequence (sync word) with higher accuracy. According toone method variant, the correlations in the second selection step areevaluated on the basis of soft-decided signal values (that is to say acorrelation result for a specific hypothetical synchronization time hasa high resolution of, for example, 16 bits and is also stored with thisresolution for subsequent evaluation). This results in the increasedaccuracy required in the second selection step.

The method according to the invention for slot synchronization of mobilestations is preferably used in the UMTS Standard. According to the UMTSSpecification 3GPP TS 25.211 V4.4.0 (2002-03), a sequence comprising 256chips is transmitted at each slot start in a first synchronizationchannel PSCH (Primary Synchronization Channel). This sequence is used todetermine the slot start in the method according to the invention.

According to another embodiment of the invention, an apparatus forsynchronization of a receiver to a transmitter has a means for repeatedcorrelation of the signal received in the receiver with the knownsequence, with correlation values of a correlation response in each casebeing produced. The apparatus further comprises a means for comparisonof the correlation response with a threshold value, a means for adaptivevariation of the threshold value as a function of at least one measuredparameter, and a first selection means for determination of possiblesynchronization times by selection of those times at which the adaptivethreshold value has been exceeded a predetermined number of times.

In addition, the apparatus includes a memory for storage of the possiblesynchronization times, and a second synchronization means which uses theset of possible synchronization times to determine the actualsynchronization time of one or more cells. The means for adaptivevariation of the threshold value varies the threshold value as afunction of the at least one measured parameter in such a way that thetotal number of possible synchronization times corresponds to apredetermined number. The adaptive variation of the threshold valuebetween successive correlations means that the number of possiblesynchronization times selected by the first selection means is matchedto the capacity of the second selection step.

One alternative embodiment of the invention is characterized by a meansfor evaluation of the correlation response and for production of the atleast one parameter, with the parameter being characteristic of thestatistical properties of the correlation response. Evaluation of thestatistical properties of the correlation responses allows goodthreshold value matching to be achieved for the stated purposes.

The means for evaluation of the correlation response and for productionof the at least one parameter as well as the means for adaptivevariation of the threshold value are, in one example, hard-wiredhardware circuits. This means that the successive threshold valuematching during successive correlations can be carried out independentlyof a processor, that is to say it does not demand any computation powerfrom a processor.

To the accomplishment of the foregoing and related ends, the inventioncomprises the features hereinafter fully described and particularlypointed out in the claims. The following description and the annexeddrawings set forth in detail certain illustrative aspects andimplementations of the invention. These are indicative, however, of buta few of the various ways in which the principles of the invention maybe employed. Other objects, advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in the following text using oneexemplary embodiment and with reference to the drawing in which:

FIG. 1 is a prior art circuit diagram illustrating a system forsynchronizing a receiver to a transmitter;

FIG. 2 is a block diagram illustrating a circuit architecture forsynchronizing a receiver to a transmitter, according to one exemplaryembodiment of the present invention;

FIG. 3 is a circuit diagram illustrating a unit configured to performinitial selection of possible synchronization times from FIG. 2;

FIG. 4 is a circuit diagram illustrating a control unit for adaptivethreshold value adjustment according to one embodiment of the invention;and

FIG. 5 is a circuit diagram illustrating a unit for determination of thestatistics obtained in the first selection step according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to FIG. 2, a circuit according to the invention for slotsynchronization has a matched filter MF, to which complex sample valuesof a synchronization signal which is transmitted via the firstsynchronization channel pSCH are supplied via an input 100. Assumingoversampling by a factor of 2, this represents 5120 sample values forthe in-phase component and 5120 sample values for the quadraturecomponent for each slot. Via an input 101, the match filter MF receivesfilter coefficients which are produced from a read only memory FC(filter coefficients). The filter coefficients are dependent on the syncword (synchronization sequence) which is transmitted via thesynchronization channel pSCH. The filter coefficients are chosen suchthat the impulse response of the matched filter MF corresponds to thecomplex-conjugate sync word reflected in time.

The matched filter MF produces a result for the real part and for theimaginary part of each sample value. The imaginary part of the filteredsignal is emitted via a data link 102, and the real part of the filteredsignal is emitted via a data link 103.

These filtered signals are then squared in the squarers SQRi and SQRr,respectively. The squared signal components are then added sample valueby sample value with the correct timings in an adder AD. A signal whichis referred to in the following text as the correlation response isemitted for one slot period at one output 104 of the adder AD. Thecorrelation response comprises 5120 correlation values for each slot.This is determined coherently, that is to say taking account of theamplitude and phase of the synchronization signal.

The correlation response is passed via a data link 105 to a firstselection unit S1 and to a second selection unit S2. The outputs of thetwo selection units S1, S2 are connected to a result memory RES-RAM,which is a volatile memory and can store 128 values. Furthermore, thetwo selection units S1 and S2 are connected via respective data links107 and 106 to two further volatile memories TEMP-RAM and MASK-RAM. Thememory TEMP-RAM in the exemplary embodiment explained here has a memorysize of 1280×16 bits, and the memory size of the memory MASK-RAM is, byway of example, 320×16 bits. As indicated by the shadow lines in FIG. 2,all of the volatile memories RES-RAM, TEMP-RAM and MASK-RAM may beduplicated.

The first selection unit S1 comprises a preselection unit PSU, astatistics unit SS (Slot Statistics) and a calculation unit MC fordetermination of variables which are stored in the memory MASK-RAM. Thesecond selection unit S2 comprises an accumulator ACC and a peakdetector PD.

The circuit illustrated in FIG. 2 operates as follows:

The preselection unit PSU is supplied with the correlation responsessuccessively via the data link 105 at an input 108. Ncor denotes thenumber of correlation responses which should be evaluated for an initialselection process. The index n indexes the correlation responses, thatis to say n=1, 2, . . . , Ncor.

During evaluation of a correlation response n in the unit PSU, eachcorrelation value of this correlation response is compared with athreshold value T(n). The index k indexes the correlation values of acorrelation response, and corresponds to the number of sample values forone slot, that is to say in the example explained here, k=1, 2, . . . ,5120.

The threshold value T(1) for the first correlation response (n=1) issignaled to the unit PSU via a digital signal processor DSP which is notillustrated. The initial threshold value is calculated in the DSP as afunction of RSSI (field strength) measurements and of the AGC setting,as well as on the basis of noise measurements, and is signaled to theunit PSU before the evaluation of the first correlation response n=1.

The preselection unit PSU compares each correlation value (relating tothe index k) of the first correlation response (n=1) with the thresholdvalue T(1). A count z₁(k)=1 is set for those correlation values whichare above the threshold value T(1).

The accumulated counts z_(n)(k) of all the previous correlationresponses including the current correlation response n relating to allthe sample values (k) for a slot are each administered in the form of4-bit words in the memory TEMP-RAM. The memory TEMP-RAM is thereforepartitioned in the form 5120×4 bits (in general: the number ofcorrelation values×the count word length). During the evaluation of then=1 subsequent correlation responses where n=2, 3, . . . , Ncor thesecounts are incremented on a sample-value related basis whenever the k-thcorrelation values of these correlation responses are in each casegreater than the associated threshold values T(n). This will bedescribed in more detail in the following text.

The correlation responses n=1, 2, . . . , Ncor are also passed via thedata link 109 to the statistics unit SS. The task of the statistics unitSS is to evaluate statistical properties of the current correlationresponse n, and then to emit a control signal (via the control line110), which is passed to the unit PSU and, if appropriate, indicates achange in the threshold value T(n+1). The changed threshold value T(n+1)is then used for evaluation of the next correlation response n+1 in theunit PSU. Once again, the individual correlation values (5120 of them)are then successively compared with the new threshold value T(n+1). Thecount z_(n+1)(k) which is administered in the memory TEMP-RAM isincremented for each sample index k, provided that the threshold valueT(n+1) is not exceeded.

After the processing of the Ncor correlation responses, the memoryTEMP-RAM thus contains the number of times that the threshold value hasbeen exceeded Z_(Ncor)(k) for each time index k. After Ncorcorrelations, Z_(NCor)(k) can thus assume the value Ncor as a maximum.

The memory contents of TEMP-RAM are then evaluated. Which of the countsZ_(Ncor)(k) (index k) and how many of them have exceeded a target count(ztar) are then determined. The target count ztar is calculated by theDSP in advance on the basis of signal strength and noise measurements.It is of course, also dependent on Ncor.

If the adaptive threshold value matching process, which will bedescribed in more detail in the following text, was successful, then thenumber of times the threshold value was exceeded with respect to thetarget count ztar is, quite accurately, 1280. In general terms, thiscorresponds highly accurately to the predetermined size of the subset ofpossible synchronization times intended for further data processing.

The memory TEMP-RAM is evaluated by the calculation unit MC. Thecalculation unit MC checks for each sample index k whether theassociated count Z_(Ncor)(k) satisfies the condition Z_(Ncor)(k)>ztar. Aflag is set to the value 1 in the memory MASK-RAM for each sample indexk for which this condition is satisfied. The memory MASK-RAM is thusused as a 1-bit memory for 5120 entries (in general: the number ofcorrelation values×1).

This therefore identifies the subset (sought in the first selectionstep) of possible candidates for the synchronization time of one or morecells. We will return later to the further data processing based on thissubset as determined in the first selection step.

FIG. 3 shows a circuit example of the design of the unit PSU. The unit108 is connected to a first input 201 of a comparator COMP. The currentthreshold value T(n) is applied to the second input 202 of thecomparator COMP.

If the correlation value entered via the input 108 is greater than T(n),the comparator COMP emits an activation signal 203. The activationsignal 203 is passed to a control input 204 of an incrementer INC. Abuffer store 206 is connected to the incrementing input 205 of theincrementer INC. The buffer store 206 contains a count z_(n−1)(k) whichhas previously been read from the memory TEMP-RAM. When an activationsignal 203 is present, the count z_(n−1)(k) is incremented by theincrementer INC. Initially, it is assumed that the incremented countz_(n)(k) is less than or equal to 15, that is to say there are noproblems in storing it as a 4-bit word in the memory TEMP-RAM. In thissituation, the value 0 is applied via a multiplexer MUX to thesubtraction input 208 of a subtractor SUB. The incremented countz_(n)(k) is received by the subtractor SUB via a data link 207 and ispassed on without being changed via a data link 209 to a saturationstage SAT which is designed for the value range (0, . . . , 15).Furthermore, the incremented count z_(n)(k) is passed to a comparator210, which checks whether z_(n)(k) assumes the value 15.

The incremented count z_(n)(k) emitted from the saturation stage SAT iswritten to the memory TEMP-RAM via the data link 107. The comparator 210as well as a control stage 211 connected downstream from the comparatorprevent the memory TEMP-RAM from overflowing in the situation where thecount z_(n)(k) has reached its maximum value 15. If the result of thecomparison process carried out in the comparator 210 for at least oneincremented count for a correlation response n is positive, the controlstage 211 switches the multiplexer MUX to the value 1 at the start ofthe evaluation of the next correlation response n+1. This results in allof the counts z_(n+1)(k) which have been supplied to the substractor SUBbeing decremented by the value 1 irrespective of whether or not theyhave been incremented in the unit INC. This ensures that the valueemitted at the output of the substractor SUB is never greater than 15.The saturation stage SAT in this case prevents negative counts frombeing produced.

In this case, the threshold value T(n) is matched for successivecorrelations n=1, 2, . . . , Ncor as follows:

The counts z_(n)(k) are passed via the data link 212 to the statisticsunit SS throughout the entire evaluation of the correlation response n.Furthermore, the statistics unit SS receives further information via thedata links 213 (correlation values), 214 (current threshold value T(n))and 215 (activation signal and threshold value decision). Once thecorrelation response n has been processed in the unit PSU, thestatistics unit SS signals via the control line 110 to the unit PSU theamount by which the previous threshold value T(n) should be changed forthe evaluation of the next correlation response n+1 (new thresholdvalue: T(n+1)).

One simple option for threshold value adaptation is explained in FIG. 3with reference to the units ACC1, a multiplexer 216, a furthermultiplexer MUX1 and a 4-value memory 217. The multiplexer 216 choosesthe value 0 for n=1, so that the accumulator ACC1 emits to thecomparator COMP the initial threshold value T(1) supplied from the DSP.The multiplexer 216 is connected to the output of the multiplexer MUX1for processing all the other correlation responses. The multiplexer MUX1has four inputs which receive the values c, −c, 1, −1. c is a positiveinteger greater than 1, which, for example, was calculated in advance bysimulations. The control signal 110 emitted from the statistics unit SSnow selects one of the four following options:T(n+1)=T(n)+1T(n+1)=T(n)−1T(n+1)=T(n)+cT(n+1)=T(n)−c

FIG. 4 shows a first section SS_1 of the statistics unit SS. The circuitsection SS_1 controls the adaptive threshold value adjustment.

An adder AD1 and a register RE1 are connected in the form of anaccumulator. The register RE1 has a reset input 301, by means of whichit is reset to the value 0 before the start of evaluation of acorrelation response. The threshold value decisions are passed to theadder AD1 via the input 215. At the end of processing of a correlationresponse, a value cand(n) is produced at the output 302 of the registerRE1, which indicates how many positive threshold value decisions havebeen made in the unit PSU during the processing of the n-th correlationresponse:cand(n)=d(k)

In this case, d(k)=1 indicates a positive threshold value decision, andd(k)=0 a negative threshold value decision.

A further distribution measure (“standard deviation”) for thecorrelation values of the n-th correlation response is calculated bymeans of a substractor SU1, a selection stage SEL, an accumulator whichcomprises a register RE2 and an adder AD2 and an optional scaling stageSCA. The subtractor SU1 forms the difference between this correlationvalue and the threshold value T(n) for each correlation value relatingto the sample k. The selection stage SEL ensures that the differencesare passed on only when they are positive, that is to say when thecorrelation value is greater than the threshold value. These positivedifferences are added up in the accumulator AD2, RE2 over the sampleindex k=1, . . . , 5120. After any scaling which may also need to becarried out by the unit SCA (the scaling is ignored in the presentexample), the further statistical measure sttdev(n) for the distributionwidth (“standard deviation”) of the correlation values of thecorrelation response n is produced.sttdev(n)=(correlation value(k)−T(n)),considering only positive expressions (correlation value (k)−T(n)).

The register RE2 is reset via a reset line 304.

The unit SCA is connected to a control circuit CON via a data line 303.The control circuit CON uses the information cand(n) and sttdev(n) todecide how the threshold value T(n+1) should be changed for the nextcorrelation response n+1. The algorithm that is provided for thispurpose is specified in the following text. In this case, a and b arefixed parameter values which have been calculated in advance bysimulation.1^(st)  case:  sttdev(n) > a  and  cand(n) > b:  T(n + 1) = T(n) + c2^(nd)  case:  sttdev(n) < a  and  cand(n) < b:  T(n + 1) = T(n) − 13^(rd)  case:  sttdev(n) > a  and  cand(n) < b:  T(n + 1) = T(n) − c4^(th)  case:  sttdev(n) < a  and  cand(n) > b:  T(n + 1) = T(n) + 1

The new threshold value T(n+1) is emitted on the basis of the above casedecision. The control line 305 presets the required parameters. The unitPSU illustrated in FIG. 3 is then controlled in the already describedmanner via the control line 110.

It is evident that the correlation response is evaluated in thestatistics unit SS on the basis of two characteristic variables,specifically a characteristic variable (sttdev(n)) for assessment of the“sharpness” of correlation peaks, and a variable (cand(n)) forassessment of the “yield”. This makes it possible to decide whether ahigh yield is due to one or more correlation peaks having been found, oris due to interference influences.

The performance of the method according to the invention is alsogoverned by suitable choice of the parameters a, b and c. In particular,the desired number of preselected possible synchronization times (inthis case 1280) can be adjusted by the choice of the parameter b.

The circuit section SS_2 illustrated in FIG. 5 can be integrated in thestatistics unit SS in order to check the initial selection step. Afterthe evaluation of all of the correlation responses, the circuit sectionSS_2 is supplied via the data link 107 with all of the final countsZ_(Ncor)(k), k=1, . . . , 5120, from the memory TEMP-RAM. The circuitsection SS_2 is activated via the control line 400 which is connected toa switch GATE.

The circuit section SS_2 has four comparators COMP1, COMP2, COMP3,COMP4, four count memories THRES1, THRES2, THRES3, THRES4, fouraccumulators AC1, AC2, AC3, AC4 and four registers R1, R2, R3, R4. Thecircuit section SS_2 allocates the counts Z_(Ncor)(k) determined overNcor correlation responses to different classes. For example, the valuesstored in the count memories THRES1, THRES2, THRES3, THRES4 may be thenumbers 1, 2, 3, 4. The comparators COMP1, COMP2, COMP3 and COMP4 alwaysemit a value 1 at their outputs 401, 402, 403, 404 whenever the valuez_(Ncor)(k) is equal to or greater than the value stored in therespective count memory THRES1, THRES2, THRES3, THRES4. The comparatoroutputs 401, 402, 403, 404 are accumulated in the respectiveaccumulators AC1, AC2, AC3, AC4, and are stored in the registers R1, R2,R3, R4.

In consequence, the register R1 contains the number of counts which areequal to or greater than the value stored in the first count memoryTHRES1, the register R2 contains the number of counts which are equal toor greater than the value stored in the count memory THRES2, etc. It isnow assumed that the target count ztar (for example ztar=3), which hasalready been defined by the DSP, is stored in the count memory THRES3.In this case, the register R3 contains the number of counts z_(Ncor)(k)greater than 3. This number is in the region of 1280, provided that thethreshold value matching has operated. The values stored in theregisters R1, R2, R3 and R4 can be read out and checked by the DSP (notshown) via an output 405. If the threshold value matching fails, it maybe necessary to change the parameter a, b or c. Between 2 and 60correlation responses are typically processed in the described mannerfor the first selection step.

The 1280 possible synchronization times found in the first selectionstep are then further restricted in the second selection step. In theexemplary embodiment described here, only a further selection ofpossible synchronization times to produce a total of 128 remainingcandidates is envisaged initially. A first option is to carry out thesecond selection step without carrying out any further correlationprocesses, solely on the basis of the data in the memory MASK-RAM. Thisis possible only when the quality of the first selection step (despitethe poor capability to differentiate between its results) issufficiently good that the best 128 synchronization times can beselected on the basis of the result class allocation process describedabove, and be stored in the memory RES-RAM.

In most cases, the second selection step includes another correlationprocess, which is more accurate and is carried out exclusively on thebasis of the possible synchronization times found in the initialselection step. This means that only correlation values at the timesstored in the memory MASK-RAM are calculated and processed further here.

At least part of the second selection step is carried out in the unitS2, which accesses the memory MASK-RAM via the data link 106 andaccumulates correlation values in the accumulator ACC at the preselectedtimes. By way of example, the correlation values are likewise calculatedin the unit MF. The correlation values are in this case determined withhigh accuracy (16 bits) and are temporarily stored in the memoryTEMP-RAM for further selection. In contrast to the first selection step,no yes/no decision is made here, with the full resolution of theaccumulated correlation values being utilized in this case. The peakdetector PD then selects from the accumulated correlation values those128 accumulated correlation values with the maximum peak height. Theseare stored in the memory RES-RAM. In the course of further processingsteps (frame synchronization, code identification), the possible timesare further restricted to the sought synchronization time, or it ispossible to provide for the sought synchronization time to be determined(as the accumulated correlation value with the maximum peak height) inthe unit S2 itself, solely with the aid of the peak detector PD.

While the invention has been illustrated and described with respect toone or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component or structurewhich performs the specified function of the described component (e.g.,that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary implementations of the invention. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in either the detailed description and the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

1. A method for synchronization of a receiver to a transmitter whichperiodically transmits a sequence which is known in the receiver,comprising: (a) determining a set of possible synchronization times in afirst selection process by repeated correlation of a signal received bythe receiver with the known sequence, resulting in a plurality ofcorrelation responses, and comparing the correlation responses with athreshold value; and (b) determining the actual synchronization timefrom the set of possible synchronization times in a second selectionprocess, wherein the threshold value is adaptively varied in the firstselection process as a function of at least one parameter that ismeasured during the correlations, and wherein the threshold value isvaried such that the total number of possible synchronization times inthe determined set corresponds to a predetermined number.
 2. The methodof claim 1, wherein determining the set of possible synchronizationtimes in the first selection process comprises: (a1) comparingcorrelation values of a correlation response with the threshold value;(a2) identifying the instances at which the correlation values of thecorrelation response exceed the threshold value; (a3) measuring the atleast one parameter; (a4) varying the threshold value as a function ofthe parameter; (a5) repeating acts (a1) to (a4) for further correlationresponses; and (a6) determining the set of possible synchronizationtimes as those times at which the adaptively varied threshold value hasbeen exceeded a predetermined number of times.
 3. The method of claim 1,wherein the at least one measured parameter is characteristic ofstatistical properties of the correlation response.
 4. The method ofclaim 3, wherein a first parameter of the at least one measuredparameter comprises a measure of the distribution width of a pluralityof correlation values associated with a respective correlation response.5. The method of claim 4, wherein a second parameter of the at least onemeasured parameter indicates the number of times at which thecorrelation values of the correlation response exceed a currentthreshold value.
 6. The method of claim 4, wherein varying the thresholdvalue comprises: increasing the threshold value by a first thresholdvalue offset when the number of times at which correlation values of thecorrelation response exceed the threshold value is greater than a firstpredetermined number; and reducing the threshold value by a secondthreshold value offset when the number of times at which the correlationvalues of the correlation response exceed the threshold value is lessthan a second predetermined number.
 7. The method of claim 6, whereinthe first threshold value offset, or the second threshold value offset,or both, increase as a distribution width of the correlation values ofthe correlation response rises.
 8. The method of claim 1, wherein acorrelation filter is employed to carry out the correlations in thefirst selection process.
 9. The method of claim 1, wherein thecorrelations are evaluated on the basis of hard-decided signal values inthe first selection process.
 10. The method of claim 1, wherein thesecond selection process includes a correlation process in determiningthe actual synchronization time.
 11. The method of claim 10, wherein acorrelation filter is employed to carry out the correlations in thesecond selection process.
 12. The method of claim 10, wherein thecorrelations are evaluated on the basis of soft-decided signal values inthe second selection process.
 13. The method of claim 1, wherein the setof possible synchronization times that are determined in the firstselection process are subdivided into classes that are distinguished bythe number of times the threshold values are exceeded.
 14. The method ofclaim 1, further comprising using the determined actual synchronizationtime for time slot synchronization of mobile stations in the UMTSstandard.
 15. An apparatus for synchronization of a receiver to atransmitter which periodically transmits a sequence that is known in thereceiver, comprising: a means for repeated correlation of the signalreceived in the receiver with the known sequence, with correlationvalues of a correlation response in each case being produced therefrom;a means for comparison of the correlation values of the correlationresponse with a threshold value; a means for adaptive variation of thethreshold value as a function of at least one measured parameter; afirst selection means for determination of possible synchronizationtimes by selection of those times at which the adaptive threshold valuehas been exceeded; and a second selection means which uses the set ofpossible synchronization times to determine the actual synchronizationtime, wherein the first selection means selects those times at which theadaptive threshold value was exceeded a predetermined number of timesduring repeated correlation, wherein the apparatus for synchronizationcomprises a memory for storage of the possible synchronization times,and wherein the means for adaptive variation of the threshold valuevaries the threshold value as a function of the at least one measuredparameter such that the total number of possible synchronization timescorresponds to a predetermined number.
 16. The apparatus of claim 15,further comprising a means for evaluation of the correlation responsesand for production of the at least one parameter, wherein the at leastone parameter is characteristic of the statistical properties of thecorrelation response.
 17. The apparatus of claim 16, wherein the meansfor evaluation of the correlation responses and for production of the atleast one parameter comprises a hard-wired hardware circuit.
 18. Theapparatus of claim 15, wherein a first parameter of the at least oneparameter comprises a measure of the distribution width of thecorrelation values of the correlation response.
 19. The apparatus ofclaim 18, wherein a second parameter of the at least one parameterindicates the number of times at which the correlation values of thecorrelation response exceed a current threshold value.
 20. The apparatusof claim 15, wherein the means for adaptive variation of the thresholdvalue comprises a hard-wired hardware circuit.
 21. The apparatus ofclaim 15, wherein the means for repeated correlation comprises acorrelation filter.
 22. The apparatus of claim 15, wherein the secondselection means comprises a correlation filter.